This invention relates to a method and apparatus for separating air. It is particularly but not exclusively concerned with separating from air an oxygen product for use in the generation at high pressure of a fuel gas which is in turn fed to the combustion chamber of a power generating gas turbine.
The operation of a gas turbine in order to generate power is well known. A gas turbine comprises a compressor, a combustion chamber and an expander. The compressor and expander are both of a rotary kind and their rotors are typically mounted on the same shaft as one another. Air is fed to the compressor and is thereby raised in pressure typically to about 15 bars. The compressed air passes to the combustion chamber in which it supports the combustion of a pressurised fluid fuel. The resulting gaseous combustion products flow into the expander and are expanded therein to a pressure of about 1 bar. The work of expansion not only provides the power necessary to drive the compressor but is also used to drive an alternator forming part of electrical power generation plant.
It is known from, for example, U.S. Pat. No. 4,224,045 and U.S. Pat. No. 4,557,735 to take a bleed of the compressed air and separate it by rectification into oxygen and nitrogen products. At least a part of the nitrogen product may be introduced into the turbine to compensate for the reduced rate of generation of combustion products that is the consequence of taking the air bleed. (Introduction of nitrogen into the combustion products also helps to reduce formation of oxides of nitrogen.) The operational output of the gas turbine depends on the flow of combustion products and nitrogen to the expander. A gas turbine is normally required to operate under a range of different conditions so as to be able to meet a range of varying demands for electrical power. In particular, demand for power during the night is usually less than in the day. Normally, the gas turbine is designed for operation at maximum output and employs an axial air compressor.
Although it is possible to turn down to some extent an axial compressor, that it is to reduce the flow rate of compressed air out of the turbine, the reduction is accompanied by a rapid drop in the outlet pressure of the air. Accordingly, turn down of the compressor to meet variations in the demand for electrical power leads to a marked reduction in the pressure at which the air is separated. Such a variable air feed pressure to the rectification column or columns of an air separation plant present major operational and control problems. It is therefore desirable to maintain a constant air feed pressure. Such a constant air feed pressure can be achieved by maintaining at steady state the operation of the air compressor forming part of the gas turbine, and appropriately increasing the rate at which air is bled to the air separation plant. However, the result of increasing the rate at which air is fed to the air separation plant is to increase the rate of oxygen production. During periods in which the electrical power demand is at a reduced level, the demand of the gas turbine for fuel gas (and hence the demand for oxygen in the gasification plant) is also reduced. Thus, the rate of production of oxygen is increased in a period when the demand for it actually falls. Accordingly, operating the air compressor at a constant pressure while varying the rate at which air is bled from it to an air separation plant will rarely be a satisfactory solution to the problem of integrating an air separation plant, a gas turbine and a gasification plant.
It is of course possible to solve these or analogous problems by having an entirely independent feed to the air separation plant. This measure however sacrifices the whole of the cost benefit that can be gained if the air separation plant is supplied from the air compressor of the gas turbine.
DE-A-3 908 505 discloses with reference to its FIG. 2 a process for separating air in which one part of the air feed is supplied from the gas turbine and another part from an independent compressor. A portion of that part of the air that is supplied from the gas turbine is condensed and is passed through a pressure reduction valve into the pressure stage of a double column comprising a first stage which operates at elevated pressure and a second stage which operates at approximately atmospheric pressure. The independent compressor also supplies air but in vapour state to the pressure column. The independent compressor thus has an outlet pressure a little above that of the pressure column. The liquid air feed to the pressure column is condensed by heat exchange with a pressurised stream of liquid oxygen withdrawn from the lower pressure column by means of a pump. The oxygen is thus vaporised. A disadvantage of this arrangement is that if nitrogen is required to be introduced from the low pressure column into the expander of the gas turbine, it is necessary to raise its pressure from about 1 bar to the operating pressure at the inlet to the expander (normally in the order of 15 bar). It is therefore not possible to obtain the substantial power savings that are achieved in for example the processes of U.S. Pat. No. 4,224,045 and U.S. Pat. No. 4,557,735 by operating the lower pressure column at a pressure well in excess of atmospheric pressure.
Indeed, if nitrogen is required to be fed to the expander of the gas turbine from the lower pressure column, DE-A-3 908 505 discloses in FIG. 3 a process for this purpose. In this process, the entire air flow to the air separation plant is taken from the air compressor of the gas turbine and the high pressure column is operated at substantially the outlet pressure of the gas turbine. In addition, an oxygen product is taken in gaseous state from the low pressure column and therefore requires compression at an oxygen outlet of the plant. Thus, this air separation plant is subject to the control problems mentioned hereinabove and also requires a gaseous oxygen compressor.
It is an aim of the present invention to provide a method and plant for separating air and producing a high pressure oxygen product which are relatively easy to control, which are able to provide an elevated pressure nitrogen stream from the lower pressure column, but which take a part of their feed from the air compressor associated with a gas turbine.